Universal Soil Loss Equation (USLE)
Table of Contents
BackgroundThe Universal Soil Loss Equation (USLE) predicts the long-term average annual rate of erosion on a field slope based on rainfall pattern, soil type, topography, crop system and management practices. USLE only predicts the amount of soil loss that results from sheet or rill erosion on a single slope and does not account for additional soil losses that might occur from gully, wind or tillage erosion. This erosion model was created for use in selected cropping and management systems, but is also applicable to non-agricultural conditions such as construction sites. The USLE can be used to compare soil losses from a particular field with a specific crop and management system to "tolerable soil loss" rates. Alternative management and crop systems may also be evaluated to determine the adequacy of conservation measures in farm planning. Five major factors are used to calculate the soil loss for a given site. Each factor is the numerical estimate of a specific condition that affects the severity of soil erosion at a particular location. The erosion values reflected by these factors can vary considerably due to varying weather conditions. Therefore, the values obtained from the USLE more accurately represent long-term averages. A calculation of soil losses using the USLE may also be done in OMAFRA's NMAN Nutrient Management Software, SOF001. The soil loss value generated from the USLE equation is used to determine the "soil erosion rating value" in the calculation of the Phosphorus Index. See the OMAFRA Factsheet Determining the Phosphorus Index for a Field, Order No. 05-067. Universal Soil Loss Equation (USLE)A = R x K x LS x C x P A represents the potential long-term average annual soil loss in tonnes per hectare (tons per acre) per year. This is the amount, which is compared to the "tolerable soil loss" limits. R is the rainfall and runoff factor by geographic location as given in Table 1. The greater the intensity and duration of the rain storm, the higher the erosion potential. Select the R factor from Table 1 based on the upper tier municipality designation and corresponding weather station where the calculation is to be made. K is the soil erodibility factor (Table 2). It is the average soil loss in tonnes/hectare (tons/acre) for a particular soil in cultivated, continuous fallow with an arbitrarily selected slope length of 22.13 m (72.6 ft) and slope steepness of 9%. K is a measure of the susceptibility of soil particles to detachment and transport by rainfall and runoff. Texture is the principal factor affecting K, but structure, organic matter and permeability also contribute. LS is the slope length-gradient factor. The LS factor represents a ratio of soil loss under given conditions to that at a site with the "standard" slope steepness of 9% and slope length of 22.13 m (72.6 ft). The steeper and longer the slope, the higher the risk for erosion. Use either Table 3A or the "Equation for Calculating LS" included in this Factsheet to obtain LS. C is the crop/vegetation and management factor. It is used to determine the relative effectiveness of soil and crop management systems in terms of preventing soil loss. The C factor is a ratio comparing the soil loss from land under a specific crop and management system to the corresponding loss from continuously fallow and tilled land. The C Factor can be determined by selecting the crop type and tillage method (Table 4A and Table 4B, respectively) that corresponds to the field and then multiplying these factors together. The C factor resulting from this calculation is a generalized C factor value for a specific crop that does not account for crop rotations or climate and annual rainfall distribution for the different agricultural regions of the country. This generalized C factor, however, provides relative numbers for the different cropping and tillage systems, thereby helping you weigh the merits of each system. P is the support practice factor. It reflects the effects of practices that will reduce the amount and rate of the water runoff and thus reduce the amount of erosion. The P factor represents the ratio of soil loss by a support practice to that of straight-row farming up and down the slope. The most commonly used supporting cropland practices are cross-slope cultivation, contour farming and strip cropping (Table 5). Procedure for Using the USLE
* Organic matter content Soil Loss Tolerance RatesA tolerable soil loss is the maximum annual amount of soil, which can be removed before the long-term natural soil productivity is adversely affected. The impact of erosion on a given soil type, and hence the tolerance level, varies, depending on the type and depth of soil. Generally, soils with deep, uniform, stone-free topsoil materials and/or not previously eroded have been assumed to have a higher tolerance limit than soils that are shallow or previously eroded. Soil loss tolerance rates are included in Table 6. The suggested tolerance level for most soils in Ontario is 6.7 tonnes/hectare/year (3 tons/acre/year) or less. Management Strategies to Reduce Soil LossesHaving obtained an estimate of the potential annual soil loss for a field, you may want to consider ways to reduce this loss to a tolerable level. Table 7 outlines management strategies to help you reduce soil erosion.
Equation for Calculation of LS (if Not Using Table 3A)LS = [0.065 + 0.0456 (slope) + 0.006541 (slope)^{2}](slope length ÷ constant)^{NN} Where: slope = slope steepness in % slope length = length of slope in m (ft) constant = 22.1 metric (72.5 Imperial) NN = see Table 3B below
Example: Calculation of Soil Erosion Using USLEA = R x K x LS x C x P Rainfall and Runoff Factor (R)The sample field is in Middlesex County. Therefore the R Factor is obtained in Table 1 from the London weather station. R Factor = 100 Soil Erodibility Factor (K)The sample field consists of fine sandy loam soil with an average organic matter content. The K Factor is obtained from Table 2. K Factor = 0.40 Slope Length-Gradient Factor (LS)The sample field is 244 m (800 ft) long with a 6% slope. The LS factor can be obtained directly from Table 3A or may be calculated using the equation on page 4. The NN value from Table 3B to be used in the equation is 0.5. LS Factor = 1.91 Crop/Vegetation and Management Factor (C)The sample field was plowed in the spring and grain corn was planted. The C Factor is obtained from the crop type factor (Table 4A) and the tillage method factor (Table 4B). Crop Type Factor for grain corn = 0.4 Tillage Method Factor for spring plow = 0.9 C Factor = 0.4 x 0.9 = 0.36 Support Practice Factor (P)Cross-slope farming is used on this sample field. The P Factor was obtained from Table 5. P Factor = 0.75 Therefore, A = R x K x LS x C x P = 100 x 0.40 x 1.91 x 0.36 x 0.75 = 20.63 tonnes/hectare/year (9.28 tons/acre/year) Referring to Table 6 in this Factsheet, you will see that this soil loss rate of 20.63 tonnes/hectare/year (9.28 tons/acre/year) is in the moderate range and considerably higher than the "tolerable loss level" of 6.7 tonnes/hectare/year (3 tons/acre/year). To reduce the soil losses for this sample field below 6.7 tonnes/hectare/year (3 tons/acre/year) we will make the following changes to the above example. Change tillage method from "spring plow (0.9)" to "no-till (0.25)" Therefore, C Factor (revised) = 0.4 x .25 = 0.10 The adjusted annual soil loss value is A = R x K x LS x C x P = 100 x 0.40 x 1.91 x 0.10 x 0.75 = 5.73 tonnes/hectare/year (2.58 tons/acre/year) Thus by changing the tillage practice, the average annual predicted soil loss for this field is below the "tolerable soil loss" of 6.7 tonnes/hectare/year (3 tons/acre/year). Additional research, experiments and data have led to the development of the Revised Universal Soil Loss Equation (RUSLE), which is a computerized version of USLE. RUSLE has the same formula as USLE, with improvements in many of the factor estimates. RUSLE can handle more complex combinations of tillage and cropping practices and a greater variety of slope shapes. A further-enhanced version of the software, known as RUSLE2, can do event-based erosion prediction. RUSLE2 requires a comprehensive set of input information, which may not be available in all jurisdictions. For more information: Toll Free: 1-877-424-1300 E-mail: ag.info.omafra@ontario.ca | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||